Piezoelectric ceramic composition and the method for preparing the same

ABSTRACT

The present invention provides a piezoelectric ceramic composition, especially a lead-free piezoelectric ceramic composition, with a suitable amount of CeO 2  or CeO 2 -containing complex additives, which is represented by the following formula: (Na 0.475 K 0.475 Li 0.05 )(Nb 1-x A x )O 3 +yCeO 2 +zBO 2  wherein “A” represents Sb or Sb+Ta, “B” represents a tetravalent transition metal, 0≦x≦0.2, 0.2 wt % &lt;y&lt;1.2 wt % and 0≦z 1 wt %. Addition of CeO 2  or CeO 2 -containing complex additives suppresses melting and abnormal grain growth of (Na 0.475 K 0.475 Li 0.05 )(Nb 1-x A x )O 3  during sintering, thereby improving sinterability and increasing the density of the composition. As a result, the dielectric and piezoelectric properties of the ceramics are enhanced. Furthermore, the represented compositions have a wide sintering temperature range so that the process for preparing such a lead-free piezoelectric ceramic composition has high degree efficiency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric ceramic composition,and particularly to a lead-free piezoelectric ceramic composition with asuitable amount of Ce0₂ or Ce0₂-containing complex additives. Also, thepresent invention relates to a method for preparing the same.

2. Background Art

In the past, as a piezoelectric ceramic composition, a Pb(Zr,Ti)0₃ (PZT)component-based ceramic containing lead has been used as the mostimportant piezoelectric compound. This is because the above PZT exhibitslarge piezoelectric properties and has a high mechanical quality factorsuperior in long-term stability and enables the easy fabrication ofmaterials of various properties required for applications such assensors, actuators, and filters. Further, PZT has a high relativedielectric constant, so can also be used as a capacitor, etc.

While a piezoelectric ceramic composition comprised of PZT has superiorproperties, it includes lead among its component elements, so harmfullead such as volatile and harmful lead oxide leaches out from theindustrial waste of products containing the PZT, and thereforemanufacture and disposal of devices using it will cause severeenvironmental pollution problem. The rising awareness of suchenvironmental issues in recent years has made difficult the productionof products that might become causes of environmental pollution such aswith PZT. Therefore, development of a piezoelectric ceramic compositioncontaining no lead is desirable.

In general, lead-free piezoelectric ceramics can be mainly divided intothree types in terms of their structures: perovskite, tungsten bronzeand bismuth layer. The lead-free piezoelectric ceramics in tungsten orbismuth layer structure are featured with high Curie temperature and areespecially suitable for high temperature applications. In perovskitetype lead-free piezoelectric ceramics, (Bi_(1/2)Na_(1/2)) Ti0₃-basedceramics (BNT) with rhombohedral perovskite structure are considered tobe good candidates to replace PZT ceramics because of its strongferroelectricity. However, as comparing with PZT ceramics, BNT ceramicshave low piezoelectric properties and low depolarization temperature. Incontrast, (Na,K)Nb0₃ (NKN)-based ceramics exhibit high Curietemperatures, large electromechanical properties and low densities,showing attractive potential for replacing PZT ceramics in somepractical use.

Although (K,Na)Nb0₃-based ceramics have comparatively excellentpiezoelectric properties, they are hard to sinter. As a result, it isfeatured with high dielectric loss, narrow sintering temperature rangeand poor reproducibility. Practically, a hot pressing technique isrequired to form a highly dense structure of (K,Na)Nb0₃ so as to giveexcellent piezoelectric properties, which results in high cost and lowefficiency in manufacture of the desired devices. The poor sinterabilityof (K, Na)Nb0₃ is due to the volatility of alkali metal components (K,Na) and a low melting point of KnbO₃, which tend to cause abnormalgrowth of grains and make it difficult to increase the sinteringtemperature to a level high enough to assure high densification of thestructure.

As a result, studies are made to improve the sinterability of(K,Na)Nb0₃-based ceramics. For example, in JP2004-244301 andJP2004-244302, it is described that the substitution of Nb by Sb andSb+Ta in the form of a solid solution improves its sinterability.However, these compositions show narrow sintering temperature range andpoor reproducibility, which is crucial for practical use. Furthermore,their piezoelectric and dielectric properties need to be improved.

SUMMARY OF THE INVENTION

The present invention has been achieved in an attempt to solve the abovementioned problems with piezoelectric ceramic compositions based on(K,Na)Nb0₃

Accordingly, an object of the present invention is to provide alead-free piezoelectric ceramic composition with a suitable amount ofCeO₂ or Ce0₂ complex additives, which shows good piezoelectric anddielectric properties as well as good reproducibility.

The present invention provides a piezoelectric ceramic compositionrepresented by the following formula:(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃+yCe0₂+zB0₂

wherein “A” represents Sb or Sb+Ta, “B” represents a tetravalenttransition metal, 0≦x≦0.2 wt % <y<1.2 wt %, and 0≦z<1 wt %, the weightpercentage y and z are calculated based on the weight of said compound(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃.

Addition of CeO₂ or Ce0₂-containing complex additives suppresses meltingand abnormal grain growth of(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃ during sintering,thereby improving sinterability and increasing the density of thecomposition. As a result, the dielectric and piezoelectric properties ofthe ceramics are enhanced.

The present invention also provides a method for preparing the abovepiezoelectric ceramic composition, comprising the steps of adding anadditive containing Ce and optional B into the compound of generalformula (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃ by the weightpercentages giving, and sintering the result to form the composition offormula (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃+yCe0₂+zB0₂,wherein “A” represents Sb or Sb+Ta, “B” represents a tetravalenttransition metal, 0≦x≦0.2, 0.2 wt % <y<1.2 wt %, and 0≦z<1 wt %, and theweight percentage y and z are calculated based on the weight of saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃.

Since the composition according to the present invention are simplyprepared by sintering a first starting material in the form of aperovskite compound represented by a formula(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃ and a second startingmaterial serving as a source of the additives yCeO₂ or yCeO₂+zB, thisprocess has high degree efficiency for preparing such a lead-freepiezoelectric ceramic composition. The process does not require anyspecial forming and sintering techniques such as the hot pressingtechnique.

Because of the improved sinterability and enhanced piezoelectricproperties, the present invention can readily be used by industries toreplace lead-containing piezoelectric ceramics in various applicationssuch as piezoelectric actuators and sensors. Piezoelectric ceramics arebeing widely used in various fields as a material for filters,actuators, transducers and any other devices capable of producing anelectric signal subjected to a mechanical excitation or a mechanicalmotion subjected to an electric excitation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features as well as other features will become apparentwith reference to the description and figures below in which likenumerals represent like elements and in which:

FIG. 1 is a SEM micrograph of the cross section of Sample I (prior art);

FIG. 2 is a SEM micrograph of the cross section of Sample 3 (the presentinvention).

FIG. 3 is a SEM micrograph of the cross section of Sample 19(comparative sample);

FIG. 4 is a SEM micrograph of the cross section of Sample 21 (thepresent invention).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, one aspect of the present invention provide apiezoelectric ceramic composition, which is represented by the followingformula:(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃+yCeO₂+zBO₂

wherein “A” can represent Sb or Sb+Ta, “B” can represent a tetravalenttransition metal, O≦x≦0.2, 0.2 wt % <y<1.2 wt %, and 0≦z≦1 wt %, saidweight percentage y and z can be calculated based on the weight of saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃.

“x can be defined as 0≦x≦0.2 since the piezoelectric ceramic compositionaccording to the present invention may or may not comprise Sb or Sb+Ta.And when Sb or Sb+Ta, is present, the suitable range of x is O<x≦0.2,because more addition of Sb or Sb+Ta leads to a deterioration inpiezoelectric and dielectric properties.

“y” can be defined as 0.2 wt % <y<1.2 wt %. If y falls outside thisrange, the sintered products are unsatisfactory. When y≦0.2 wt %,addition of CeO₂ has no significant effect on the sinterability. On theother hand, when y≧1.2 wt %, there is significant decrease in both thedielectric and piezoelectric properties.

“z” can be defined as 0≦z<1 wt %, since the piezoelectric ceramiccomposition according to the present invention may comprise or may notcomprise a complex additive consisting of CeO₂ and B0₂. When such acomplex additive is presented, the suitable range for “z” is 0<z<1 wt %,because addition of BO2 more than 1 wt % is not favorable forimprovement of piezoelectric properties.

Preferably, “B” is at least one element selected from the groupconsisting of Mn, Ti and Zr.

Further, the range of y and z preferably meet the relational expressionof 0.2 wt % <y+z≦2.0 wt %. Especially, y is preferable to fall into therange of 0.4 wt % ≦y≦0.8 wt %. Alternatively or additionally, x andymeet the relational expressions of 0<x≦0.08 and 0≦z≦0.6 wt %respectively. More preferably, 0.4 wt % ≦z≦0.6 wt %.

In one embodiment of this invention, “A” is not in existence (x=0), andthe piezoelectric composition is represented by a formula(Na_(0.475)K_(0.475)Li_(0.05))Nb0₃+yCeO₂+zB0₂. In this case, addition ofCe0₂ or Ce0₂+B0₂ leads to an increase in sintering temperature and adecrease in grain size, and improves the piezoelectric properties of(Na_(0.475)K_(0.475)Li_(0.05))Nb0₃ ceramics. And the preferred range of“y” is 0.4 wt % ≦y≦0.8 wt %. Illustrative examples of such compositioninclude (Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.4 wt %CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.6 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.8 wt % CeO₂; and the like. Toimprove both dielectric and piezoelectric properties, a longer soakingtime is recommended. However, the temperature range for well-sinteredCeO₂-added (Na_(0.475)K_(0.475)Li_(0.05))NbO₃ composition is stillnarrow, and its dielectric properties are not optimal, although it isstill superior to the traditional piezoelectric compound(Na_(0.475)K_(0.475)Li_(0.05))NbO₃.

So, in a preferred embodiment of this invention, “A” is present (i.e.x≠0), but “B” is not present (i.e. z=0), and the piezoelectriccomposition is represented by a formula(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃+yCeO₂. In this case, thedielectric and piezoelectric properties of such composition are superiorto those without addition of additives, i.e.(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃ (“A” represents Sb orSb+Ta). This clearly shows the effects of CeO₂ to improve dielectric andpiezoelectric properties of the composition. The preferred range of y is0.4 wt % ≦y≦0.8 wt %, and the preferred range of x is 0<x≦0.08. Examplesof such composition include

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.99)Sb_(0.01))O₃+0.4 wt % CeO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.99)Sb_(0.01))O₃+0.8 wt % CeO₂;

(Na_(0.475)K_(0.475))Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.8 wt % CeO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.4 wt %CeO₂; and the like.

In a further preferred embodiment of this invention, both “A” and “B”are in existence (x≠0, z≠0), and the piezoelectric composition isrepresented by a formula(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃+yCeO₂+zBO₂. In thiscase, such compositions have dielectric and piezoelectric propertiessuperior not only to (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(l-x)A_(x))0₃ butalso to (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))0₃+yCeO₂ (“A”represents Sb or Sb+Ta). This indicates that the CeO₂-containing complexadditives are more effective in improving the sinterability anddielectric and piezoelectric properties of the composition than the CeO₂additive. The preferred y and z meet the relational expression of 0.2 wt% <y+z≦2.0 wt %. More preferably, the range of y is 0.4 wt % ≦y≦0.8 wt%, the preferred range of x is 0<x≦0.08, and the preferred range of z is0<z≦0.6 wt %. Examples of such composition include

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % MnO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.6 wt % CeO₂+0.6wt % MnO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % TiO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03)O₃)O₃+0.6 wt % CeO₂+0.6wt % TiO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % ZrO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.6 wt % CeO₂+0.6wt % ZrO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.4 wt %CeO₂+0.4 wt % MnO₂;

(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.6 wt %CeO₂+0.6 wt % MnO₂; and the like.

In another preferred embodiment of this invention, “B” represents Mn,0<x≦0.2, and 0<z<1 wt %. That is to say, when using CeO₂+MnO₂ asCeO₂-containing complex additives, the composition should comprises Sbor Sb+Ta as “A”.

The other aspect of the invention provides a method for preparing apiezoelectric ceramic composition, comprising the steps of adding anadditive containing Ce and optional B into the compound of generalformula (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ by the weightpercentages giving, and sintering the result to form the composition offormula (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃+yCeO₂+zBO₂wherein “A” represents Sb or Sb+Ta, “B” represents a tetravalenttransition metal, 0≦x≦0.2, 0.2 wt % <y<1.2 wt %, O≦z<1 wt %, and saidweight percentage y and z are calculated based on the weight of saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃.

In one embodiment of the process, the compound(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ is prepared by mixing acompound containing sodium, a compound containing potassium, a compoundcontaining sodium, a compound containing niobium, a optional compoundcontaining tantalum, and an optional compound containing antimony by astoichiometric ratio giving, and calcining the mixture to get saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃.

Preferably, the compound containing sodium is Na₂CO₃, said compoundcontaining potassium is K₂CO₃, said compound containing lithium isLi₂CO₃, said compound containing niobium is Nb₂O₅, said compoundcontaining tantalum is Ta₂O₅ and said compound containing antimony isSb₂O₅ and said additive is CeO₂ and optional B0₂ “B” is at least oneelement selected from the group consisting of Mn, Ti and Zr.

In the lead-free piezoelectric composition of the present inventiondescribed above, Ce may enter into the crystal structure or exist at thegrain boundary. The additives serve to restrict melting and grain growthof (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃, thereby promotingthe sinterability of the perovskite compound at a sintering or firingtemperature in a range, in which the amount of volatilization of alkalicomponents is sufficiently small. In particular, partial substitution ofNb by Sb or Sb+Ta reduces the total content of KNbO₃ (which has acomparatively low melting point), thereby raising the melting point ofthe system as a whole, making it possible to raise the sinteringtemperature to a level at which the volatilization of the alkalicomponents does not matter, so that the sinterability of the perovskitecompound can be further improved. Namely, the present lead-freepiezoelectric ceramic composition can be well sintered in a widertemperature range and in a normal manner, without employing a specialforming and sintering technique such as the hot pressing technique. Thethus improved sinterability results in intended densification of thesintered body, which enhances the piezoelectric properties. Inparticular, the partial substitution of Nb by Sb or Sb+Ta results in aconsiderable improvement of the dielectric and piezoelectric propertiesof the sintered body. Thus, the present lead-free piezoelectric ceramiccomposition exhibits excellent piezoelectric properties and has a highdegree of efficiency of preparation or fabrication.

EXAMPLES

The present invention will next be described in detail by way ofexamples while referring to the drawings, but the examples should notconstructed as limiting the invention thereto.

Example I

Starting materials for this example can be Na₂CO₃, K₂CO₃, Li2CO₃, Nb₂O₅and CeO₂.

These materials were weighed to give a composition of(Na_(0.475)K_(0.475)Li_(0.05))NbO₃. The thus-weighed materials werewet-milled in a ball mill for about 10 hours, and the resulting mixturedried and then calcined at 800 to 950° C. Then CeO₂ in different amountsas shown in Table 1 added into the calcined mixture, and the resultingmixture was wet-milled and dried again.

Each of the thus-dried mixture granulated and added with PVA as abinder. Then, each of the granulated powders was pressed under apressure of 1000 kg/cm² to obtain circular discs in a diameter of 12 mmand a thickness of 1.4 mm. These discs are sintered at a temperaturebetween 1070 and 1140° C. in the usual manner to obtain sintered ceramicdiscs.

A silver paste applied to both surfaces of the sintered ceramic discsand fired to form silver electrodes. Then, they poled under an electricfield of 5 kV/mm to 10 kV/mm for 15 to 30 minutes in silicon oil at 130to 180° C. After poling, a d meter (ZJ-30 PIEZO d₃₃ METER) used tomeasure the d₃₃ coefficient at 100 Hz. The resonance measurementsperformed using an impedance analyzer such as one sold under the brandname by Hewlett-Packard 4294A Impedance Analyzer by Hewlett-Packard, aDelaware Corporation. The electromechanical coupling factor kp of thevibration in the radial direction was calculated from the resonance andthe anti-resonance frequencies according to Onoe's formulas. Dielectricproperties were determined using an impedance analyzer such as one soldunder the brand name Hewlett-Packard 4192A Impedance Analyzer byHewlett-Packard at 1 kHz. The data obtained are shown in Tables I-2 andFIGS. 1-2. TABLE 1 Dielectric and piezoelectric properties of Ce02-added(Na^(0.475)K_(0.475)Li_(0.05))NbO₃ ceramics Sintering Dielectric LossSample Amount of condition k_(p) d₃₃ Constant tanσ No. Ce0₂(wt %) (° C.zh) (%) (pC/N) ε_(r) (%) 1 0 1080 × 2 36 141 454 11.5 2 0.2 1090 × 2 34132 624 6.4 3 0.4 1090 × 2 40 148 785 2.2 4 0.6 1110 × 2 45 198 1013 2.15 0.8 1120 × 2 42 182 981 1.4 6 1.2 1120 × 2 35 137 884 2.1

TABLE 2 Dielectric and piezoelectric properties of the(Na_(0.475)K_(0.475)Li_(0.05))NbO₃ + 0.8 wt % CeO₂ composition sinteredat 1120° C. for different soaking times. Sintering Dielectric Loss tanσcondition Kp(%) D33 constant ε_(r) (%) 1120 × 2 42 182 981 1.4 1120 × 1046 194 1238 3.2

In Table 1, samples No. 3 to No. 5 are within the scope of theinvention. For sample No. 2, there is no significant effect on thesinterability by the addition of 0.2 wt % CeO₂, so it is out the scopeof the invention. On the other hand, probably due to the formation oftoo much defects, there is significant decrease in both the dielectricand piezoelectric properties by the addition of 1.2 wt % CeO₂, so sampleNo. 6 is also out the scope of the invention. As shown in Table 1,samples No. 3 to No. 5 show dielectric properties superior to those ofsample No. 1 (without addition of CeO₂ one of piezoelectrics in theprior art). The addition of CeO₂ leads to an increase in sinteringtemperature and a decrease in grain size (FIGS. 1-2), implying that CeO₂suppresses the grain growth effectively. It is also seen that by theaddition of a suitable amount of CeO₂ the piezoelectric properties of(Na_(0.475)K_(0.475)Li_(0.05))NbO₃ ceramics can be improved.

In addition, as shown in Table 2, a longer soaking time is helpful forimproving both the dielectric and piezoelectric properties. However, thetemperature range for well-sintering Ce0₂-added(Na_(0.475)K_(0.475)Li_(0.05))NbO₃ composition is still narrow, andtheir dielectric properties are not good enough and needed to be furtherimproved.

Example 2

In a second example,the starting materials wereNa₂CO₃,K₂CO₃,Li₂CO₃,Nb₂O₅,Sb₂O₅,Ta₂O₅,CeO₂, and MnO₂.

These materials were weighed to give a composition of(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃+yCeO₂+zB where x, y andz are as in Table 3 below. These materials were processed in the samemanner as in Example 1 to obtain piezoelectric ceramic disc samples.

Measurements were conducted to obtain the relative dielectric constant(ε_(r)) of these disk samples, electromechanical coupling factor k ofthe vibration in the radius direction, and piezoelectric constant d Thedata obtained are shown in Tables 3-4. TABLE 3 Dielectric andpiezoelectric properties of the composition(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ + yCeO₂ + zBO₂ SinteringDielectric Loss Sample y z condition k_(p) d₃₃ constant tanδ No. A B x(wt %) (wt %) (° C. × h) (%) (pC/N) ε_(r) (%) 7 Sb — 0.01 0 0 1070 × 229 131 522 2.0 8 Sb — 0.03 0 0 1080 × 2 37 147 686 6.3 9 Sb — 0.01 0.4 01110 × 2 40 151 822 3.5 10 Sb — 0.01 0.8 0 1120 × 2 41 164 990 2.2 11 Sb— 0.03 0.4 0 1100 × 2 38 155 702 6.4 12 Sb — 0.03 0.8 0 1110 × 2 42 1741014 4.5 13 Sb Mn 0.03 0.4 0.4 1100 × 2 43 184 890 3.0 14 Sb Mn 0.03 0.60.6 1120 × 2 42 193 1047 2.2 15 Sb Ti 0.03 0.4 0.4 1100 × 2 43 187 9852.7 16 Sb Ti 0.03 0.6 0.6 1100 × 2 40 171 936 3.1 17 Sb Zr 0.03 0.4 0.41100 × 2 42 196 1093 2.5 18 Sb Zr 0.03 0.6 0.6 1100 × 2 41 180 1016 2.319 Sb + Ta — 0.03 + 0.05 0 0 1080 × 2 45 207 1166 2.1 20 Sb + Ta —0.03 + 0.05 0.4 0 1090 × 2 45 211 1175 2.2 21 Sb + Ta Mn 0.03 + 0.05 0.40.4 1100 × 2 45 223 1223 1.8 22 Sb + Ta Mn 0.03 + 0.05 0.6 0.6 1110 × 243 197 1114 2.1

TABLE 4 Dielectric and piezoelectric properties of the compositionsintered at different temperatures(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃ + 0.4 wt %CeO₂ + 0.4 wt % MnO₂ Sintering Dielectric Loss condition d₃₃ constanttanδ Density ρ (° C. × h) k_(p) (%) (pC/N) ε_(r) (%) (g/cm³) 1090 × 2 44207 1091 2.1 4.38 1100 × 2 45 223 1223 1.8 4.49 1110 × 2 45 214 1204 1.94.46 1120 × 2 44 216 1239 1.8 4.41

In Table 3, samples No. 9 to No. 18 and samples No. 20 to No.22 arewithin the scope of the invention.

As shown in Table 3, samples No. 9 to No.12 all have dielectric andpiezoelectric properties superior to those of samples No. 7 to No. 8(without addition of additives), which are the conventional ceramics(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)Sb_(x))O₃. This clearly shows theeffects of CeO₂ to improve the dielectric and piezoelectric propertiesof the composition. It can also be seen that samples No. 13 to No. 18have dielectric and piezoelectric properties superior not only to thoseof samples No. 7 and No.8, but also to samples No. 9 to No. 12. Thisindicates that the CeO complex additives are more effective in improvingthe sinterability and dielectric and piezoelectric properties of thecomposition than the CeO₂ additive.

Likewise, as comparing the dielectric and piezoelectric properties ofsamples No. 19 to No. 22, it can be seen that the CeO₂ andCe0₂-containing complex additives have similar effects on the(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)Sb_(x))O₃ composition. Inparticular, as shown in Table 4, the Ce0₂-containing complex additivescan also improve the sintering temperature range for the composition.

Similar to the cases in Example 1, addition of the Ce0₂ complexadditives can suppress the grain growth effectively (see FIGS. 3-4).

It has been confirmed that the total amount of the additives is definedto be not larger than about 3% by weight. Preferably the amount is lessthan 2%.

It has been found that Mn0₂ functions well together with CeO₂ in thecomposition containing Sb such as samples No. 13, No. 14, No. 21 and No.22. In fact, addition of Mn0₂ in the(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ composition with no Sbdoes not raise or even lower the sintering temperature and decrease theelectric properties. From this, it is understood that the effect of Mn0₂may concern with the existence of Sb in the(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ composition, which iswithin the scope of the invention.

While the invention has been described with reference to specificembodiments chosen for purpose of illustration, it should be apparentthat numerous modifications could be made thereto by those skilled inthe art without departing from the basic concept and scope of theinvention. Such variations are intended to be within the scope of thepresent invention.

1. A piezoelectric ceramic composition represented by the followingformula:(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃+yCeO₂+zBO₂ wherein “A”represents Sb or Sb+Ta, “B” represents a tetravalent transition metal,0≦x≦0.2, 0.2 wt % <y<1.2 wt %, and 0≦z<1 wt %, said weight percentage yand z are calculated based on the weight of said compound(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃.
 2. A piezoelectricceramic composition according to claim 1, wherein “B” is at least oneelement selected from the group consisting of Mn, Ti and Zr.
 3. Apiezoelectric ceramic composition according to claim 1 or 2, wherein 0.2wt % <y+z≦2.0 wt %.
 4. A piezoelectric ceramic composition according toclaim 1 or 2, wherein 0.4 wt % ≦y≦0.8 wt %.
 5. A piezoelectric ceramiccomposition according to claim 4, wherein O<x≦0.08, and 0≦z≦0.6 wt %. 6.A piezoelectric ceramic composition according to claim 5, wherein O.4 wt% ≦z≦0.6 wt %.
 7. A piezoelectric ceramic composition according to claim1 or 2, wherein “B” represents Mn, O<x≦0.2, and O<z<1 wt %.
 8. Apiezoelectric ceramic composition according to claim 1 or 2, whereinsaid piezoelectric ceramic composition is selected from the list of:(Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.4 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.6 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))NbO₃+0.8 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.99)Sb_(0.01))O₃+0.4 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.8 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.8 wt % CeO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % MnO₂; (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.6 wt %CeO₂+0.6 wt % MnO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % TiO₂; (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.6 wt %CeO₂+0.6 wt % TiO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.4 wt % CeO₂+0.4wt % ZrO₂; (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.97)Sb_(0.03))O₃+0.6 wt %CeO₂+0.6 wt % ZrO₂;(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.4 wt %CeO₂; (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.4wt % CeO₂+0.4 wt % MnO₂;and(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(0.92)Ta_(0.05)Sb_(0.03))O₃+0.6 wt %CeO₂+0.6 wt % MnO₂.
 9. A method for preparing a piezoelectric ceramiccomposition, comprising the steps of: adding an additive containing Ceand optional B into the compound of general formula(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ by the weightpercentages giving, and sintering the result to form the composition offormula (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃+yCeO₂+zBO₂,wherein “A’ represents Sb or Sb+Ta, “B” represents a tetravalenttransition metal, 0≦x≦0.2, 0.2 wt % <y<1.2 wt %, and 0≦z<1 wt %,saidweight percentage y and z are calculated based on the weight of saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃.
 10. A methodaccording to claim 9, wherein said compound(Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃ is prepared by: mixing acompound containing sodium, a compound containing potassium, a compoundcontaining lithium, a compound containing niobium, a optional compoundcontaining tantalum, and a optional compound containing antimony by astoichiometric ratio giving, and calcining the mixture to get saidcompound (Na_(0.475)K_(0.475)Li_(0.05))(Nb_(1-x)A_(x))O₃.
 11. A methodaccording to claim 10, wherein said compound containing sodium isNa₂CO₃, said compound containing potassium is K₂CO₃, said compoundcontaining lithium is Li₂CO₃, said compound containing niobium is Nb₂O₅,said compound containing tantalum is Ta₂O₅, and said compound containingantimony is Sb₂O₅ and said additive is CeO₂ and optional B0₂ “B” is atleast one element selected from the group consisting of Mn, Ti and Zr.